This invention relates to magnetic fluid.
Among magnetic fluids are known dispersions of fine particles of magnetic oxide material, typically magnetite (Fe.sub.3 O.sub.4) pretreated with a surface-active agent in oil or water. Such magnetic fluids have been utilized in a variety of applications.
Known magnetic fluids of such magnetic materials as magnetite can only provide a saturation magnetization of the order of 200 to 300 G and in special cases, 550 to 600 G at the maximum. The relatively low saturation magnetization is a bar against more useful application of magnetic fluids. There is the need for development of magnetic fluid having high saturation magnetization.
The saturation magnetization of magnetic fluid may be enhanced by two processes, that is, by increasing the saturation magnetization of dispersed fine particles themselves and by increasing the content of fine particles in the medium. The latter process of increasing the concentration of fine particles, however, cannot produce a magnetic fluid having a saturation magnetization in excess of 600 G with magnetite because the magnetic fluid undesirably becomes paste with the increasing concentration.
The former magnetic fluids using fine particles having high saturation magnetization are described in J. Thomas, U.S. Pat. No. 3,228,881 and O. Harle, U.S. Pat. No. 3,228,882, for example. Cobalt carbonyl Co.sub.2 (CO).sub.8 or iron carbonyl Fe(CO).sub.5 is thermally decomposed in a polymer such as acrylonitrile-styrene copolymer and then dispersed in a hydrocarbon medium. Since fine particles having high saturation megnetization are employed in these methods, the resulting magnetic fluids are expected to have higher saturation magnetization than the magnetite magnetic fluids. It is, however, difficult to increase the concentration of fine particles because the resulting fluid becomes paste. For this reason, actually prepared magnetic fluids possess only a saturation magnetization of the order of 300 to 400 G, which value is not of significance as compared with the magnetite magnetic fluids.
Other known methods includes the preparation of ferromagnetic fine particles of Fe, Ni, Co or the like by an electroless plating process followed by dispersion in a suitable medium, and the preparation of fine particles of Fe, Co or the like by the explosion of such material in an inert gas followed by dispersion in a suitable medium. None of these methods have succeeded in producing magnetic fluid having desirably high saturation magnetization.
Metal fine particles in these magnetic fluids are expected to be readily oxidized with a probable reduction in saturation magnetization.